1. Field of the Invention
The present invention relates to a method for fabricating an electronic device including an infrared sensor, for example, and an electronic device to be preferably fabricated by such a method.
2. Description of the Related Art
An infrared sensor, including a plurality of bolometers on a semiconductor substrate, is known in the art. The infrared spectral responsivity of such an infrared sensor decreases when the heat, generated in the bolometers responsive to incident infrared radiation, is transmitted to the semiconductor substrate. Thus, to ensure sufficient infrared spectral responsivity, it is necessary to decrease the thermal transferability between the bolometers and the semiconductor substrate. For that purpose, Japanese Laid-Open Publication No. 2001-210877 discloses a technique of creating a cavity on the surface of a silicon substrate to thermally isolate the silicon substrate with a huge heat capacity from infrared detectors such as bolometers.
Hereinafter, the technique disclosed in Japanese Laid-Open Publication No. 2001-210877 mentioned above will be described with reference to FIGS. 31A through 31G. According to the conventional method, first, as shown in FIG. 31A, the surface of a silicon substrate 1001 is thermally oxidized locally to form a locally oxidized silicon (LOCOS) film 1002 thereon.
Next, as shown in FIG. 31B, a silicon nitride layer 1003 and a polysilicon film 104 are stacked in this order over the LOCOS film 1002 and the silicon substrate 1001.
Thereafter, as shown in FIG. 31C, a plurality of holes 1005 are formed by photolithographic and dry etching processes so as to extend through the polysilicon film 1004, silicon nitride layer 1003 and LOCOS film 1002 and reach the surface of the silicon substrate 1001.
Subsequently, as shown in FIG. 31D, portions of the LOCOS film 1002, which are exposed on the inner surfaces of the holes 1005, are removed laterally by a wet etching process using buffered hydrofluoric acid. As a result, walls 1007 are defined by the remaining portions of the LOCOS the 1002 between the adjacent holes 1005.
Next, as shown in FIG. 31E, a thin polysilicon film is deposited on the surface of the discontinued polysilicon film 1004 and on the inner surfaces of the holes 1005 and then the thin polysilicon film and the discontinued polysilicon film 1004 are oxidized together to form a continuous silicon dioxide layer 1010. As a result of this process step, the holes 1005 are closed up with the silicon dioxide layer 1010 to define cavities 1011 as closed spaces.
Thereafter, as shown in FIG. 31F, a patterned conductor film 1012 with a zigzag planar shape, for example, is deposited on the silicon dioxide layer 1010 so as to function as an infrared detector.
By providing the cavities 1011 between the conductor film 1012 as a heat detector and the silicon substrate 1001 in this manner, the transfer of the heat from the infrared detector to the silicon substrate 1001 can be reduced, thus increasing the infrared spectral responsivity.
Hereinafter, another method for creating the cavities will be described. An infrared sensor, including cavities formed by such a method, is disclosed in Japanese Laid-Open Publication No. 05-126643, for example.
First, as shown in FIGS. 32A and 32B, a silicon dioxide layer 301 is deposited on a silicon substrate 300. When a polysilicon film to be deposited in the next process step is etched, the silicon dioxide layer 301 will function as a lower etch stop layer.
Next, as shown in FIGS. 33A and 33B, a polysilicon film 302 is deposited on the silicon dioxide layer 301 and then patterned as shown in FIGS. 34A and 34B. The patterned polysilicon film 302 will function as a sacrificial layer to be etched away to form a cavity.
Subsequently, as shown in FIGS. 35A and 35B, another silicon dioxide layer 303 is deposited on the polysilicon film 302 and then an infrared detector 304 is formed on the silicon dioxide layer 303 as shown in FIGS. 36A and 36B.
Thereafter, as shown in FIGS. 37A and 37B, yet another silicon dioxide layer 305 is deposited over the infrared detector 304. These silicon dioxide layers 303 and 305 function as an upper etch stop layer.
Then, as shown in FIGS. 38A and 38B, the silicon dioxide layers 303 and 305 are patterned to define cavity forming openings 306. Portions of the polysilicon film 302 are exposed at the bottom of these openings 306. Subsequently, hydrazine is introduced through the openings 306 of the silicon dioxide layers 303 and 305, thereby etching the polysilicon film 302. In this manner, a cavity 308 is formed as shown in FIGS. 39A and 39B.
In the method disclosed in Japanese Laid-Open Publication No. 2001-210877, the walls 1007 remain between the adjacent cavities 1011 as shown in FIG. 31F. To increase the effects to be obtained by providing the cavities 1011, the walls 1007, having some thermal conductivity, are preferably removed. The walls 1007 may be removed by performing the etching process step shown in FIG. 31D long enough to leave no walls 1007 there. However, if the walls 1007 were removed at this early stage, then the silicon nitride layer 1003 and the polysilicon film 1004 would crack before the structure shown in FIG. 31F is completed. Such a phenomenon is believed to be caused by a thermal stress resulting from a difference in thermal expansion coefficient between the silicon nitride layer 1003 and the silicon substrate 1001. That is to say, while the conductor film 1012 of polysilicon is annealed to activate a dopant that has been introduced into the conductor film 1012 and while the polysilicon film 1004 and the thin polysilicon film are thermally oxidized, a great thermal stress will be applied to the silicon nitride layer 1003 and silicon dioxide layer 1004.
For that reason, according to the method disclosed in Japanese Laid-Open Publication No. 2001-210877, it is difficult to form a big cavity by removing the walls 1007.
According to the method disclosed in Japanese Laid-Open Publication No. 05-126643 on the other hand, the polysilicon film 302 is removed by a chemical agent such as hydrazine, thus always requiring a drying process step to remove the chemical agent from the cavity 308. However, when such a drying process step is carried out, a great stress is created in the portions of the silicon dioxide layers 303 and 305 that support the ceiling of the cavity 308, thus possibly cracking those silicon dioxide layers 303 and 305.
In order to overcome the problems described above, preferred embodiments of the present invention provide an electronic device, in which members defining the ceiling of a cavity are not cracked, and a method for fabricating such an electronic device.
A method for fabricating an electronic device according to a preferred embodiment of the present invention preferably includes the steps of: (a) preparing a cavity defining sacrificial layer, at least the upper surface of which is covered with an etch stop layer; (b) forming at least one first opening in the etch stop layer, thereby partially exposing the surface of the cavity defining sacrificial layer; (c) etching the cavity defining sacrificial layer through the first opening, thereby defining a provisional cavity under the etch stop layer and a supporting portion that supports the etch stop layer thereon; and (d) etching away a portion of the etch stop layer, thereby defining at least one second opening that reaches the provisional cavity through the etch stop layer and expanding the provisional cavity into a final cavity.
In one preferred embodiment, the step (d) preferably includes the step of etching at least a part of the supporting portion, which is located under the second opening, through the second opening.
In another preferred embodiment of the present invention, the method preferably further includes the step of forming a structure, including a patterned thin film, on the etch stop layer before the step (d) is performed.
In this particular preferred embodiment, the step of forming the structure preferably includes the step of forming the structure such that the patterned thin film does not overlap with the portion of the etch stop layer to be removed to define the second opening in the step (d).
In another preferred embodiment, the step (a) preferably includes the steps of: depositing a material film of the cavity defining sacrificial layer on a substrate; and patterning the material film into the shape of the cavity defining sacrificial layer.
In this particular preferred embodiment, the step of patterning the material film preferably includes the step of patterning the material film into a cavity defining sacrificial layer that has a through hole extending from the upper surface thereof through the lower surface thereof.
Specifically, the step (c) preferably includes the step of defining the supporting portion in a region in which the cavity defining sacrificial layer is not present.
More specifically, the step (c) preferably includes the step of making a portion of the etch stop layer function as the supporting portion.
In another preferred embodiment, the step (c) preferably includes the step of leaving a portion of the cavity defining sacrificial layer as the supporting portion.
In still another preferred embodiment, the step (c) preferably includes the step of selectively removing the cavity defining sacrificial layer by a wet etching technique, and the step (d) preferably includes the step of removing the supporting portion at least partially by a dry etching technique.
In yet another preferred embodiment, the step (a) preferably includes the step of depositing the etch stop layer on the cavity defining sacrificial layer.
In yet another preferred embodiment, the step (a) preferably includes the step of preparing an SOI substrate that includes a silicon dioxide layer functioning as the etch stop layer and a single crystalline silicon substrate including a portion functioning as the cavity defining sacrificial layer.
In yet another preferred embodiment, the method preferably further includes the steps of: defining a mask, having a pattern that will define the second opening and that exposes the inside of the first opening, on the etch stop layer between the steps (b) and (c); and removing the mask after the step (d) has been performed.
In yet another preferred embodiment, the method preferably further includes, between the steps (c) and (e), the steps of: depositing a thin film on the etch stop layer to close up the first opening of the etch stop layer; forming a film for a sensor on the thin film; and patterning the film for the sensor.
In this particular preferred embodiment, the step of depositing the thin film preferably includes the step of depositing the thin film by a chemical vapor deposition process.
In that case, the method preferably further includes the step of forming a heat-absorbing insulating film on the thin film.
Then, the method preferably further includes the step of forming a passivation film on the heat-absorbing insulating film.
In yet another preferred embodiment, the step (a) preferably includes the step of locally oxidizing the surface of a single crystalline silicon substrate to define a silicon dioxide region on a selected area on the surface of the silicon substrate. In that case, at least a portion of the silicon dioxide region is preferably used as the cavity defining sacrificial layer.
In this particular preferred embodiment, the method preferably further includes the step of using the silicon dioxide region as an isolation film.
In yet another preferred embodiment, the step (a) preferably includes the step of using a surface portion of a semiconductor substrate as the cavity defining sacrificial layer.
In yet another preferred embodiment, the step (c) preferably includes the steps of: forming a recess, extending from the first opening into the cavity defining sacrificial layer, by a dry etching technique; and expanding the recess by an isotropic etching technique.
In yet another preferred embodiment, the step (c) may include the step of defining the supporting portion only around the provisional cavity.
In an alternative preferred embodiment, the step (c) may include the step of defining the supporting portion only inside of the provisional cavity.
In yet another preferred embodiment, where the final cavity has an overall transversal sectional area of about 1,000 xcexcm2 or more, the step (c) preferably includes the step of defining three to ten columns, each having a transversal sectional area of at least about 10 xcexcm2, as the supporting portion.
In yet another preferred embodiment, the step (a) preferably includes the step of depositing a nitride layer as the etch stop layer, and the step of depositing the thin film preferably includes the step of depositing a silicon dioxide film.
In yet another preferred embodiment, the method preferably further includes the step of forming a cap member that encapsulates the structure including the patterned thin film.
An electronic device according to a preferred embodiment of the present invention preferably includes: a substrate with at least one cavity; a thin film structure, which defines the upper surface of the cavity; and a patterned thin film that is supported by the thin film structure. In this electronic device, the thin film structure preferably includes at least one hole, which is not overlapped by the patterned thin film and which reaches the cavity.
In one preferred embodiment of the present invention, a convex portion may be provided inside of the cavity and right under the hole so as to protrude toward the thin film structure.
In an alternative preferred embodiment, a concave portion may be provided inside of the cavity and right under the hole so as to protrude away from the thin film structure.
In another preferred embodiment, the patterned thin film is preferably a bolometer, and the electronic device preferably functions as an infrared sensor.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.